The present invention relates to a semiconductor device that has an active region of a crystalline silicon film obtained by crystallizing an amorphous silicon film and a method for fabricating the device. The present invention is effective, in particular, for a semiconductor device that employs a thin film transistor (TFT) provided on a substrate having an insulating surface and is able to be utilized for an active matrix type liquid crystal display device, a contact-type image sensor, a three-dimensional IC and the like.
In recent years, there have been attempts to form a high-performance semiconductor device on an insulating substrate of glass or an insulating film for the achievement of a large-size high-resolution liquid crystal display device, a high-speed high-resolution contact-type image sensor, a three-dimensional IC and the like. It is general to use thin-film-shaped silicon semiconductors as semiconductor elements for use in these devices. The silicon semiconductors can be roughly categorized into the two types of an amorphous silicon semiconductor (a-Si) and a silicon semiconductor having crystallinity.
The amorphous silicon semiconductor, which can be relatively easily fabricated by the vapor deposition method at a low fabricating temperature and suitable for mass production, is most generally used. However, the amorphous silicon semiconductor has inferior physical properties of electrical conductivity and so on with respect to the silicon semiconductor having crystallinity. Therefore, in order to obtain a higher-speed characteristic, it is strongly demanded to establish a method for fabricating a semiconductor device made of a silicon semiconductor having crystallinity. As a silicon semiconductor having crystallinity, there are known polysilicon, microcrystal silicon, amorphous silicon including a crystalline component and semi-amorphous silicon having a state intermediate between the crystalline property and the amorphous property.
As a method for obtaining a silicon semiconductor having crystallinity, there are known the following methods (1) through (3).
(1) A method for directly forming a film having crystallinity in a film-forming stage
(2) A method for forming an amorphous semiconductor film and crystallizing the film by the energy of laser light
(3) A method for forming an amorphous semiconductor film and crystallizing the film by applying thermal energy
However, according to the aforementioned method (1), crystallization progresses concurrently with the film forming process. Therefore, in order to obtain a silicon film having crystals of a large grain size, it is indispensable to increase the thickness of the silicon film. It is technically difficult to uniformly form a silicon film having satisfactory semiconductor properties thoroughly on the entire surface of the substrate.
According to the aforementioned method (2), which utilizes the crystallization phenomenon through a melting and solidifying process, the grain boundaries are satisfactorily processed although the grain size is small, allowing a high-quality crystalline silicon film to be obtained. However, taking the excimer laser, which is currently most generally used, as an example, it is difficult to uniformly process the entire surface of a large-area substrate since the laser stability is not sufficient.
According to the aforementioned method (3), which is advantageous in terms of uniformity and stability inside the substrate by comparison with the methods (1) and (2), is used for a microminiature high-definition LCD panel employing a quartz substrate. However, in this case, after growing crystals through heat treatment at a temperature of 600xc2x0 C. for a long time of about 30 hours, the crystals are further subjected to heat treatment at an high temperature of about 1000xc2x0 C. for several tens of minutes to several hours for the promotion of improvement in crystallinity, and this results in a prolonged processing time and a degraded throughput. If a TFT is fabricated with this crystallized silicon film, then only the device characteristic of a field effect mobility of about 100 cm2/Vs can be obtained.
In order to solve the aforementioned problems, a method of an improvement of the aforementioned method (3) is disclosed in Japanese Patent Laid-Open Publication No. HEI 7-335905. This Japanese Patent Laid-Open Publication No. HEI 7-335905 discloses a reduction in heating temperature, a reduction in processing time and an improvement in crystallinity by utilizing a catalytic element that promotes the crystallization of the amorphous silicon film.
Specifically, a trace quantity of a metallic element of nickel, palladium or the like is introduced into the surface of the amorphous silicon film, and thereafter heat treatment is performed. With regard to the mechanism of low-temperature crystallization, the generation of a crystalline nucleus occurs in the early stage with the metallic element serving as the nucleus. Subsequently, the metallic element serves as a catalyst to promote the crystal growth for the rapid promotion of the crystallization, according to the understanding. In this sense, the aforementioned metallic element is referred to as a xe2x80x9ccatalytic elementxe2x80x9d hereinafter. By comparison with the twin crystal structure in one particle of the crystalline silicon film crystallized by the normal solid phase epitaxy, the crystalline silicon film that has undergone crystal growth with the catalytic element forms an aggregate of a number of columnar crystals, each of which has an almost ideal monocrystalline internal state.
According to the method of the aforementioned Japanese Patent Laid-Open Publication No. HEI 7-335905, crystals grow sidewise (in a direction parallel to the substrate) from the region into which the catalytic element is introduced by crystallizing only the region into which the catalytic element is selectively introduced with the other portion remaining in a state of an amorphous silicon film through selective introduction of the catalytic element into part of the amorphous silicon film and heating of the same and further prolonging the heating time.
Furthermore, according to the method of the aforementioned Japanese Patent Laid-Open Publication No. HEI 7-335905, the sidewise crystal growth distance is increased by forming the amorphous silicon film that is the start film to be crystallized by the low pressure CVD method, and the active region of the thin film transistor is formed by using the sidewise crystal growth region.
In contrast to this, the Japanese Patent Laid-Open Publication No. HEI 7-307286 discloses another example of the use of the catalytic element, or the use of the catalytic element for dehydrogenation in the amorphous silicon film.
Specifically, the dehydrogenating reaction in the amorphous silicon film is promoted by introducing the catalytic element into the amorphous silicon film containing hydrogen and heating the film at a low temperature of not higher than 550xc2x0 C., and thereafter, the amorphous silicon film is crystallized by laser light application. The dehydrogenation in the amorphous silicon film is thus promoted and thereafter the amorphous silicon film is crystallized by laser light application. Therefore, the film exfoliation is hard to be caused by rapid dehydrogenation of the silicon film even if the laser light application is performed, and a crystalline silicon film having uniform crystallinity is obtained.
As disclosed in the aforementioned Japanese Patent Laid-Open Publication No. HEI 7-335905 and the Japanese Patent Laid-Open Publication No. HEI 7-307286, the crystalline silicon film obtained by introducing the catalytic element into the amorphous silicon film and crystallizing the film through heat treatment is excellent in terms of crystallinity. In particular, the crystal orientation is closer to the conventional monocrystal than the polycrystal. Through confirmation with a TEM (Transmission Electron Microscope), the present inventor has obtained a distinct diffraction pattern that exhibits a monocrystal state even in a wide range of selected area of a diameter of 25 xcexcm.
The above is because a crystalline silicon film fabricated with a catalytic element has undergone a definitely different peculiar crystal growth from that of a crystalline silicon film fabricated by the conventional solid phase epitaxy method that has a very low nucleus incident rate and causes only the temporary generation of a crystalline nucleus. Therefore, in the crystalline silicon film fabricated with the catalytic element, the crystal grain grown from each crystalline nucleus grows very large to have a grain size of up to 10 xcexcm to 30 xcexcm. The crystal particle is normally called the grain. However, there exist a number of more minute particles inside the crystal grain grown as a crystal from an identical nucleus, and therefore, the crystal grain is referred to as a domain hereinafter.
If a plurality of TFT""s are fabricated with such a crystalline silicon film, then the particle diameter of each domain is very large and randomly exist. Accordingly, there are included TFT""s fabricated inside the domain and TFT""s fabricated across the grain boundaries. Therefore, if a TFT is fabricated inside the domain, a high-performance TFT is obtained. However, if a TFT is fabricated across the grain boundary, the TFT has an inferior ability. This consequently causes a problem that the TFT characteristics (mobility, threshold voltage value, etc.) vary within the crystalline silicon film.
As proposed in Japanese Patent Laid-Open Publication No. HEI 7-335905, this problem seems to be simply solved by the method of selectively introducing the catalytic element into the amorphous silicon film and growing a crystal sidewise from the region into which the catalytic element has been introduced. However, according to this method, the characteristic variation becomes more significant. The above is because nuclei randomly occur in the region into which the catalytic element has been introduced before the sidewise crystal growth, and the domain grown from each nucleus merely progresses further sidewise. Therefore, domains elongated in the direction of growth exist along the direction of growth in the region to be used as a TFT active region, and therefore, it is sometimes the case where a TFT is fabricated across the grain boundary. This consequently varies the TFT characteristics.
It is to be noted that the domain mentioned in connection with the present invention emerges as a stepped portion by etching the Si surface with hydrofluoric acid or oxidizing the Si surface, and the domain can also be confirmed as a difference in contrast by an optical microscope.
Accordingly, the object of the present invention is to provide a stable high-performance semiconductor device having little characteristic variation and a high-performance semiconductor device having a high integration density. Another object is to provide a semiconductor device fabricating method capable of simply fabricating such a semiconductor device with high yield.
In order to achieve the above object, there is provided a semiconductor device comprising a plurality of thin film transistors formed with a silicon film having crystallinity serving as an active region on a substrate having an insulating surface,
the silicon film being a polysilicon including a catalytic element for promoting crystallization of the silicon film itself, and the silicon film having a constituent crystal grain size smaller than a size of a channel region of each of the thin film transistors.
The silicon film crystallized with the catalytic element has a crystalline structure of an aggregate of columnar crystals from a microscopic viewpoint as described above. The silicon film structure itself reduces the defects inside the crystal grain. If the silicon film is used for the active regions (source/drain region and channel region) of the thin film transistor, then a high current driving ability is provided. In order to reduce the characteristic variation between elements while keeping this high current driving ability, the grain boundary is required to be controlled. However, it is theoretically required to perform control at the microscopic level, which is impossible.
Therefore, according to the semiconductor device of the above constitution, it has been discovered that the above object can be achieved by constructing the active regions of a plurality of thin film transistors of the crystalline silicon film including the catalytic element for promoting the crystallization and setting the crystal grain size of each crystal in the crystalline silicon film smaller than the size of the channel region of each thin film transistor. That is, by setting the size of crystal grain constituting the crystalline silicon film smaller than the size of the channel region formed in the thin film transistor, the characteristic variation of the thin film transistor can be remarkably improved.
In particular, the active matrix substrate for liquid crystal display use or the like is required to have higher characteristic uniformity since the characteristic variations of the thin film transistors for driving the pixel electrodes appear as a display unevenness. Therefore, by applying the present invention to the active matrix substrate or the like, a device of sufficient display quality can be obtained, and a driver monolithic structure in which a driver circuit is concurrently formed in the peripheral portion can be provided.
In one embodiment of the present invention, the constituent crystal grain size of the silicon film is not greater than 5 xcexcm.
In this constitution, the crystal grain size of each polysilicon including the catalytic element should preferably be not greater than 5 xcexcm. It has been discovered that the characteristic variations of the normally formed thin film transistors can be more surely reduced if the crystal grain size is within this range.
If the crystal grain size of the silicon film is greater than 5 xcexcm, then the thin film transistors are sometimes fabricated across the grain boundaries, increasing the characteristic variations of the thin film transistors.
In one embodiment of the present invention, the catalytic element is the element of nickel and the concentration of the element of nickel in the silicon film is within a range of 1xc3x971016 to 1xc3x971018 atoms/cm3.
In this constitution, the catalytic element to be contained in the silicon film that constitutes the active regions of the thin film transistors should preferably be the element of nickel. That is, it has been discovered that the element of nickel can form a silicon film of the best crystallinity among catalytic elements.
It has also been discovered that a silicon film of satisfactory crystallinity can be formed by setting the catalytic element concentration in the silicon film to 1xc3x971016 to 1xc3x971018 atoms/cm3.
If the catalytic element concentration in the silicon film is lower than 1xc3x971016 atoms/cm3, then the effect of sufficient crystal growth cannot be obtained since the concentration is less than the solubility limit.
If the catalytic element concentration in the silicon film becomes higher than 1xc3x971010 atoms/cm3, then the device characteristics receive bad influence to cause an increase in leak current or the like.
Also, there is provided a semiconductor device fabricating method comprising:
a process for forming an amorphous silicon film containing hydrogen on a substrate having an insulating surface;
a process for adding to a surface of the amorphous silicon film a catalytic element for promoting crystallization of the amorphous silicon film; and
a process for crystallizing the amorphous silicon film by subjecting the amorphous silicon film to which the catalytic element has been added to heat treatment.
According to the semiconductor device fabricating method, the amorphous silicon film containing hydrogen is formed on the substrate having the insulating surface, and thereafter, the catalytic element for promoting the crystallization is added to the amorphous silicon film containing hydrogen. If the amorphous silicon film to which the catalytic element has been added is subjected to heat treatment, then the crystal growth starts from the upper surface side of the amorphous silicon to crystallize the amorphous silicon film. As a result, the crystallized silicon film has a satisfactory crystalline structure of a columnar crystal network structure, and the grain size of individual crystal becomes minute. That is, the intended crystalline silicon film of the present invention is obtained. By employing such a crystalline silicon film, the TFT characteristic variation inside the substrate is improved to allow a semiconductor device having excellent characteristic uniformity to be provided.
Also, there is provided a semiconductor device fabricating method comprising:
a process for adding a catalytic element for promoting crystallization of an amorphous silicon film to a substrate having an insulating surface;
a process for forming the amorphous silicon film containing hydrogen on the surface which belongs to the substrate and to which the catalytic element has been added; and
a process for crystallizing the amorphous silicon film by subjecting the amorphous silicon film to heat treatment.
According to the semiconductor device fabricating method, the catalytic element for promoting the crystallization of the amorphous silicon film is added to the substrate having an insulating surface, and thereafter, the amorphous silicon film containing hydrogen is formed on the surface of the substrate to which the catalytic element is added. The, upon subjecting the amorphous silicon film to heat treatment, crystal growth starts from the lower surface side of the amorphous silicon to crystallize the amorphous silicon film. As a result, the crystallized silicon film has a crystalline structure of a satisfactory columnar crystal network structure, and the grain size of individual crystal becomes minute. That is, the intended crystalline silicon film of the present invention is obtained. By employing such a crystalline silicon film, the TFT characteristic variation inside the substrate is improved to allow a semiconductor device having excellent characteristic uniformity to be provided.
In one embodiment of the present invention, the hydrogen concentration in the amorphous silicon film is within a range of 3 to 25 atomic percent.
The semiconductor device fabricating method of the embodiment is based on the following phenomenon discovered by the present inventor. According, to the examination carried out by the present inventor, it has been discovered that the hydrogen in the amorphous silicon film exerts a great influence on the crystal form. That is, the crystal grain size of the crystalline silicon film obtained through the heat treatment after the addition of the catalytic element drastically changes across a specified value depending on the concentration of hydrogen contained in the amorphous silicon film.
FIG. 9 is a graph showing a relation between the hydrogen concentration in the initial amorphous silicon film and the crystal grain size of the silicon film crystallized by the catalytic element. In this case, the element of nickel is used as the catalytic element. Despite that the concentration of the element of nickel is set identical (5xc3x971012 atoms/cm2 by the concentration in the surface after addition), as shown in FIG. 9, the crystal grain size abruptly increases when the hydrogen concentration is reduced to a value under a specified value. This threshold value is about 3 to 5 atomic %.
FIG. 10A is a view of an optical microscope photograph of the surface state of a crystalline silicon film obtained by crystallizing an amorphous silicon film having a hydrogen concentration of not higher than 3 atomic %. As shown in FIG. 10A, if the hydrogen concentration in the amorphous silicon film is not higher than 3 atomic %, then a very large crystal grain that exceeds a grain size of 30 xcexcm is observed. FIG. 10B is a view of an optical microscope photograph of the surface state of a crystalline silicon film obtained by crystallizing an amorphous silicon film having a hydrogen concentration of about 10 atomic %. FIG. 10A and FIG. 10B have an identical magnification ratio. If the hydrogen concentration in the amorphous silicon film is about 10 atomic %, then the crystal grain is very small, and as shown in FIG. 10B, it is very difficult to confirm the crystal grain size by an optical microscope. According to the observation by TEM, the crystal grain size was about 1 to 2 xcexcm.
As described above and apparent from the curve of the graph of FIG. 9, the crystal growth states shown in FIGS. 10A and 10B have quite different growth modes rather than on the extension of change in hydrogen concentration. Then, the threshold value across which the growth mode changes exists within the hydrogen concentration range of 3 to 5 atomic %. If the hydrogen concentration in the amorphous silicon film is lower than the threshold value of 3 atomic %, then the growth from each nucleus is great and a gigantic crystal grain is formed although the density of occurrence of crystalline nuclei is extremely reduced. If the hydrogen concentration in the amorphous silicon film exceeds the threshold value of 5 atomic %, then the growth from each nucleus is little and the crystal grain becomes minute although the density of occurrence of crystalline nuclei is extremely increased. In either case, the density of occurrence of crystalline nuclei is changed by the concentration of the added catalytic element. However, the changes are limited in the respective modes, and a decisive difference depending on the change in hydrogen concentration in the amorphous silicon film can be observed at whatever catalytic element concentration.
Of course, the addition of the catalytic element should preferably be little considering the influence on the semiconductor. However, if the addition of the catalytic element is little, then an uncrystallized region remains in the crystalline silicon film as shown in FIGS. 10A and 10B, degrading the semiconductor characteristics. How the uncrystallized region remains largely depends on the difference in hydrogen concentration in the amorphous silicon film. In the case of the low hydrogen concentration in FIG. 10A, the uncrystallized region remains in a large region in correspondence with the giganticness of the crystal grain. Strangely enough, the occurrence of a crystalline nucleus in this uncrystallized region is not observed at all. In the case of the high hydrogen concentration in FIG. 10B, where the crystalline nuclei are minute and the density of occurrence of the crystalline nuclei is high, if the uncrystallized region remains at the same areal rate as that in the case of FIG. 10A, then the individual uncrystallized regions are small and uniformly dispersed inside the film. Not only the nonuniformity simply attributed to the crystal grain but also how the uncrystallized region remains exert a great influence on the semiconductor characteristic variation. Particularly when it is desired to restrain the catalytic element, the low hydrogen concentration is not permitted in terms of uniformity. In contrast to this, when the hydrogen concentration is within the range of 3 to 25 atomic %, as proposed by the present invention, there is no problem in terms of device characteristic uniformity even if the uncrystallized region remains a little so long as the uncrystallized region is crystallized through the subsequent heat treatment.
The mechanism of the occurrence of the above phenomenon is not clearly discovered. Although conjectural, the following mechanism can be considered. The catalytic element has already been diffused in a metal state in the amorphous silicon film before the crystallization (during temperature increase) of the amorphous silicon film. The catalytic element in the metal state causes reaction in the amorphous silicon film to form a silicide. In the silicified location, the crystallization of the amorphous silicon film progresses. The crystallization will be described depending on cases in which the hydrogen concentration is low and is high.
If the hydrogen concentration in the amorphous silicon film is low, taking the state of occurrence of the crystalline nucleus in the amorphous silicon film into consideration, the added catalytic element moves in the amorphous silicon film to cause crystallization, and therefore, aggregates in excess of a specified amount are formed in spots here and there. Then, each of these aggregates becomes a nucleus to form a gigantic crystal grain. Therefore, almost no catalytic element exists between the adjoining nuclei, and no new nucleus occurs there.
If the hydrogen concentration in the amorphous silicon film is high, it is presumed that hydrogen is preventing the movement of the catalytic element. Consequently, the catalytic element contributes to the crystallization in the originally added state without movement. As a result, crystalline nuclei uniformly occur in the amorphous silicon film, and minute crystal particles are uniformly formed without growing large since the quantity of catalytic element in each crystalline nucleus is small.
Therefore, as described above, in order to achieve the objects of the present invention, the hydrogen concentration in the amorphous silicon film that contains hydrogen is required to be not lower than 3 atomic %. In contrast to this, the upper limit of the hydrogen concentration is based at least on the condition that the film exfoliation of the silicon film should not be caused by the release of a large amount of hydrogen from the silicon film through the heat treatment for the crystallization of the amorphous silicon film. From this point of view, the hydrogen concentration should preferably be not higher than 25 atomic %. Therefore, by setting the hydrogen concentration in the amorphous silicon film within a range of 3 to 25 atomic %, the occurrence of crystalline nuclei such that minute crystal particles are uniformly formed is caused to allow the characteristic variation of the thin film transistor to be more surely reduced. If the hydrogen concentration in the amorphous silicon film is lower than 3 atomic %, then it is sometimes the case where a gigantic crystal grain is formed and a thin film transistor is fabricated across the grain boundary, causing the characteristic variation of the thin film transistors. If the hydrogen concentration in the amorphous silicon film exceeds 25 atomic %, then the film exfoliation is caused by the release of a large amount of hydrogen from the silicon film, exerting a bad influence on the device characteristics of the thin film transistors.
In one embodiment of the present invention, the amorphous silicon film is formed by a plasma CVD method at a heating temperature of not higher than 400xc2x0 C.
According to the semiconductor device fabricating method of the embodiment, by forming an amorphous silicon film at a heating temperature of not higher than 400xc2x0 C. by the plasma CVD method, the hydrogen concentration in the amorphous silicon film can be formed roughly uniformly within the aforementioned specified range, and this allows the method to be applied also to a large-area substrate with good repeoducibility.
In one embodiment of the present invention, the element of nickel is used as the catalytic element.
According to the semiconductor device fabricating method of the embodiment, it has been discovered that a crystallization promoting effect can be produced with a trace quantity of one sort or a plurality of sorts of the elements of Ni, Co, Pd, Pt, Cu, Ag, Au, In, Sn, Al and Sb and, in particular, the most remarkable crystallization promoting effect can be obtained when nickel (Ni) is employed. The reason for the above can be given by the following model. The element of nickel does not singly effect but produces the crystal growth effect by being silicified through bonding with the amorphous silicon film. The crystalline structure in the above stage effects as a sort of mold when the amorphous silicon film is crystallized, promoting the crystallization of the amorphous silicon film. The element of nickel forms a silicide of NiSi2 reacting with two silicon (Si) elements. This NiSi2 has a fluorite type crystal structure, and the crystalline structure bears a close resemblance to the diamond structure of single crystal silicon. Furthermore, the aforementioned NiSi2 has a lattice constant of 5.406 xc3x85, which is very close to the lattice constant of 5.430 xc3x85 of the diamond structure of the crystal silicon. Therefore, the aforementioned NiSi2 is the ultimate one as a mold for crystallizing the amorphous silicon film and able to mostly promote the crystallization of the amorphous silicon film.
In one embodiment of the present invention, after the addition of the element of nickel to the surface of the amorphous silicon film or the surface of the substrate, the nickel concentration of the surface is within a range of 1xc3x971012 to 5xc3x971013 atoms/cm
According to the semiconductor device fabricating method of the embodiment, it has been discovered that setting the nickel concentration on the surface to 1xc3x971012 to 5xc3x971013 atoms/cm2 after the addition of nickel to the surface of the amorphous silicon film or the surface of the substrate is preferable in forming a silicon film of satisfactory crystallinity.
If the nickel-concentration is lower than 1xc3x971012 atoms/cm2, then the amorphous silicon film is not crystallized since the minimum concentration that causes the crystallization is not lower than 1xc3x971012 atoms/cm2.
If the nickel concentration exceeds 5xc3x971013 atoms/cm2, then the influence of nickel remaining in the grain boundaries increases to exert a bad influence on the semiconductor characteristics. Specifically, the leak current in the turning-off stage increases in the thin film transistor, and this causes the characteristic variations of the thin film transistors.
In one embodiment of the present invention, the process for adding the element of nickel to the surface of the amorphous silicon film or the surface of the substrate is performed by spin coating a nickel solution on the surface of the amorphous silicon film or the surface of the substrate.
According to the semiconductor device fabricating method of the embodiment, the method of spin coating a nickel solution on the surface of the amorphous silicon film or the surface of the substrate is effective as a method for adding the element of nickel to the surface of the amorphous silicon film or the surface of the substrate.
According to this method, by performing spin coating with a solution in which nickel is dissolved, the nickel concentration in the solution is controlled to enable the trace quantity control of nickel to be introduced into the surface of the amorphous silicon film or the surface of the substrate.
In the solution in which nickel is dissolved according to the above method, it is preferable to employ acetate or nitrate of nickel as a solute and employ an alcohol-based material of ethanol or isopropyl alcohol (IPA) as a solvent. By employing a solution constructed of the above solute and solvent, a stabilized crystal growth can be obtained on the entire surface of the amorphous silicon film. Particularly on a large-size substrate of a liquid crystal display device or the like, excellent film quality can be obtained with uniformity throughout the entire substrate surface.
In one embodiment of the present invention, the process for adding the element of nickel to the surface of the amorphous silicon film or the surface of the substrate is performed by DC sputtering the element of nickel at a low voltage.
According to the semiconductor device fabricating method of the embodiment, by adding nickel to the surface of the amorphous silicon film or the surface of the substrate by DC sputtering at a low voltage, very good uniformity is obtained and particles and the like scarcely cause the nonuniformity of crystal growth although the method is inferior to the spin coating method in terms of trace control at a low concentration.
The reason why the term of xe2x80x9cadditionxe2x80x9d is intentionally used instead of xe2x80x9cfilm formationxe2x80x9d is that the sputtering is performed by such a trace quantity that it is insufficient for film formation.
In one embodiment of the present invention, the heating process for crystallizing the amorphous silicon film has a first step for releasing excessive hydrogen existing in the amorphous silicon film and a second step intended for crystal growth of the amorphous silicon film.
According to the semiconductor device fabricating method of the embodiment, the heat treatment for crystallizing the amorphous silicon film containing hydrogen should preferably be performed in the two steps of the first-step heat treatment for releasing the excessive hydrogen existing in the amorphous silicon film and the second-step heat treatment intended for the crystal growth in the amorphous silicon film. This is because the above arrangement allows the elimination of film exfoliation that might occur when the heat treatment intended for the achievement of crystallization is abruptly performed, by performing the first-step heat treatment and thereafter performing the second-step heat treatment intended for crystallization.
In this case, the crystallization performed by extracting hydrogen from the amorphous silicon film seems to be contradictory to the purport of the present invention. However, the hydrogen concentration in the amorphous silicon film in the catalytic element adding stage is important according to the present invention, and the dehydrogenating process performed after the catalytic element addition does not matter. Although the reasons for the above have not been clearly discovered, there has been obtained the reality that the dehydrogenating process performed after the catalytic element addition does not matter also through experiments.
In one embodiment of the present invention, the heating process of the first step is performed within a temperature range of 450xc2x0 C. to 530xc2x0 C., and the heating process of the second step is performed within a temperature range of 530xc2x0 C. to 650xc2x0 C.
According to the semiconductor device fabricating method of the embodiment, the first-step heat treatment should preferably be performed within the temperature range of 450xc2x0 C. to 530xc2x0 C. in reducing the hydrogen concentration in the amorphous silicon film. The above is because the temperature range of 400 to 450xc2x0 C. is the boundary considered from the energy of bonding of hydrogen to silicon and also from the experimental evidence, and the effective dehydrogenating effect can be practically obtained only at a temperature of not lower than 450xc2x0 C. That is, if the first-step heat treatment is performed at a temperature lower than 450xc2x0 C., then no effective dehydrogenating effect can be obtained on the amorphous silicon film containing hydrogen. If the heat treatment temperature in the first step becomes higher than 530xc2x0 C., then the dehydrogenation rate is increased, as a consequence of which the film exfoliation of the silicon film tends to easily occur. Furthermore, if the heat treatment temperature in the first step becomes higher than 530xc2x0 C., then the crystallization of the amorphous silicon film is started by the effect of the catalytic element, and therefore, the temperature in the first step is required to be not higher than 530xc2x0 C. Therefore, the hydrogen in the amorphous silicon film is required to be released slowly and sufficiently through the first-step heat treatment.
The second-step heat treatment should preferably be performed within the temperature range of 530xc2x0 C. to 650xc2x0 C. in crystallizing the amorphous silicon film. The temperature that causes the crystallization of the amorphous silicon film is required to be not lower than 530xc2x0 C. That is, if the temperature in the second step is lower than 530xc2x0 C., then the amorphous silicon film cannot be crystallized. However, if the temperature is excessively high, then the crystallization rate is so fast that the crystal defects of dislocation and the like frequently occur. Therefore, the upper limit of the temperature in the second step becomes about 650xc2x0 C. That is, if the heating temperature in the second step exceeds 650xc2x0 C., then the crystal defects of dislocation and the like frequently occur.
In one embodiment of the present invention, the semiconductor device fabricating method further comprises a process, which is to be performed after the crystallization of the amorphous silicon film through heat treatment, for moving most of the catalytic element remaining in the crystallized silicon film toward a region outside a region in which a semiconductor element is to be formed.
The semiconductor device fabricating method of the present invention is considerably characterized in that the amorphous silicon film is crystallized by the catalytic element. However, even with a trace quantity of addition, the existence of such a metal element in the semiconductor film itself is unfavorable.
Therefore, the semiconductor device fabricating method of the embodiment prevent the catalytic element from exerting a bad influence on the semiconductor element with the provision of the process for moving most of the catalytic element remaining in the silicon film to a region other than the semiconductor element forming region after the catalytic element is utilized for the crystallization process of the amorphous silicon film.
In one embodiment of the present invention, the semiconductor device fabricating method further comprises a process, which is to be performed after the crystallization of the amorphous silicon film through heat treatment, for further promoting the crystallinity of the silicon film by applying laser light to the crystallized silicon film.
According to the semiconductor device fabricating method of the embodiment, the method of crystallizing the amorphous silicon film containing hydrogen through heat treatment and thereafter further promoting the crystallinity of the silicon film by the application of laser light to the crystallized silicon film is effective as a method for improving the semiconductor device performance or, in particular, the current drive ability by improving the crystallinity of the silicon film crystallized by the catalytic element. The reasons for the above will be described below.
When intense light of laser or the like is applied to the crystallized silicon film, the grain boundaries are intensively processed due to the difference in melting point between the crystalline silicon film and the amorphous silicon film. However, in the crystalline silicon film formed by the normal solid phase epitaxy method, the crystalline structure is in the twin crystal state, and therefore, the inside of the crystal grain remains as a twin crystal defect after the application of intense light. In contrast to the above, the crystalline silicon film crystallized by the catalytic element is formed of columnar crystals, and the inside of the crystalline silicon film is in a single crystal state. Therefore, if the grain boundaries are processed by the application of intense light, then a crystalline silicon film of a good quality close to the single crystal state throughout the entire surface is obtained. That is, the method of crystallizing the amorphous silicon film containing hydrogen through heat treatment and thereafter further promoting the crystallinity of the silicon film by the application of laser light to the crystallized silicon film is very effective in terms of crystallinity. Laser light is applied to the silicon film that originally has crystallinity, and therefore, variation in laser light application is largely alleviated in contrast to the method of performing crystallization by directly applying laser light to the amorphous silicon film, causing no problem about uniformity.
The laser light to be used in this stage has a very high absorption coefficient with respect to the silicon film so long as the wavelength is not longer than 400 nm, allowing only the silicon film to be instantaneously heated without giving any thermal damage to the substrate of glass or the like. In particular, the XeCl excimer laser having a wavelength of 308 nm has a great output. Therefore, the beam size at the time of application to the substrate can be increased, and this allows the laser to easily cope with a large-area substrate. The output is relatively stabilized, which is desirable for the application to a mass-production apparatus. By applying the above-mentioned laser light to the surface of the silicon film in a manner that the radiation energy density of the laser light becomes 200 to 450 mJ/cm2, the crystallinity of the silicon film crystallized by the catalytic element is promoted to allow a crystalline silicon film of a very high quality to be obtained. If the radiation energy density of the laser light is smaller than 200 mJ/cm2, then the silicon film is scarcely melted, as a consequence of which the promotion of crystallinity cannot sufficiently be achieved. If the radiation energy density is greater than 450 mJ/cm2, then the crystallinity obtained by the catalytic element is completely lost or reset, causing the problem of nonuniformity observed in the conventional laser crystallization.
In one embodiment of the present invention, the semiconductor device fabricating method further comprises a process, which is to be performed after the crystallization of the amorphous silicon film through heat treatment, for further promoting the crystallinity of the silicon film by subjecting the crystallized silicon film to heat treatment at an elevated temperature higher than the heat treatment temperature in an oxidizing atmosphere.
According to the semiconductor device fabricating method of the embodiment, the method of crystallizing the amorphous silicon film containing hydrogen through heat treatment and thereafter further promoting the crystallinity by performing heat treatment in an oxidizing atmosphere at a temperature higher than the heat treatment temperature is also effective as another method for improving the crystallinity of the silicon film crystallized by the catalytic element to further improve the performance, or, in particular, the current drive ability of the semiconductor device. If the silicon film crystallized by the catalytic element is subjected to heat treatment in an oxidizing atmosphere at a high-temperature (for example, 800xc2x0 C. to 1100xc2x0 C.), then oversaturated Si atoms caused by the oxidizing operation are supplied to the silicon film. As a result, the oversaturated Si atoms enter the crystal defects (in particular, dangling bond) in the silicon film, allowing the defects to be removed. Therefore, the density of defects in the silicon film crystallized by the catalytic element is remarkably reduced and the mobility is remarkably improved, allowing the semiconductor device performance to be remarkably improved.